1. Field of Invention
The invention relates to a receiving structure of a data communication system. More particularly, the invention relates to an adaptive equalizer provided on the front end of a receiver that compensates for signal attenuation and signal degradation in the receiving end of a data communication system.
2. Related Art
Data communications refer to the information transmission on a communication medium. For example, the communication medium is air in a wireless communication system, optical fibers in an optical communication network, and coaxial cables in a wire communication system. The transmitted information on such communication media can be digital or analog data.
In a wire communication system, the data transmission of an ordinary wire communication device requires a transmitter 1. The transmitter 1 is connected to a receiver 2 through a transmission cable 3. As shown in FIG. 1, the Ethernet has a transmission cable 3 shorter than 100 m. In an equivalent circuit of FIG. 1, the transmission cable 3 can be viewed as a low-pass filter. Therefore, the low frequency threshold of the transmission cable 3 will lose data. For example, a long continuous and identical transmission signal is in fact a low frequency analog signal, which is cut off by transformers during transmissions. To alleviate this margin effect, the transmitted signal pulses should avoid low frequency components as possible. However, the transmitted signal on the transmission cable 3 still contain low frequency components actually, which results in low frequency degradation on the receiver 2, so that the received signals have the so-called baseline wander effect.
The baseline wander effect refers to the phenomenon that the ground level of the transmission cable 3 deviates from the ideal ground level. When the baseline wander effect happens, the data transmitted on the transmission cable 3 contains DC components. Therefore, the front end of the receiver 2 is further installed a baseline wander compensator to compensate for the signal losses. In the prior art, the baseline wander compensator always detects the incoming signal from the transmission cable 3 and performs baseline wander compensations.
Moreover, to couple different transmission cables 3 to the receiver 2, the front end of the receiver 2 also contains an equalizer with gain control to provide appropriate amplitude and/or phase compensations for the incoming signals. According to the prior art, the equalizer of the receiving end takes a long T period of time as its observation unit, which is usually 500 ns and the worst case is 57 bit time, as shown in FIG. 2. When the high frequency signal is not good, the equalizer still uses long T as its observation unit which is not able to effectively compensate for the high frequency component. The signal jitters become worse. Furthermore, the prior art simultaneously activates an equalizer and a baseline wander compensation circuit at the front end of the receiver 2. After the incoming signals are detected, the equalizer and the baseline wander compensation circuit are simultaneously enabled for operations, so that the two compensation devices work independently. Therefore, the gain control of the equalizer using long T as its observation unit and the compensation effects of the baseline wander compensator also using long T as its observation unit may cancel with each other and are thus unable to achieve the expected compensation effects.
Accordingly, the invention provides a receiver that determines when the equalizer and the baseline wander compensator should perform compensations for incoming signals by judging whether the incoming signal amplitude reaches a predetermined threshold and a compensation method used on the receiver. Thus, the invention can increase the efficiency of the equalizer and the baseline wander compensator.